This is the National Library of Medicine's free search engine for "biomedical" literature, but in practice it includes all major general science journals, anything related to molecular biology, many more general biology journals (weaker on ecology etc.), and in general gobbs of evolution stuff on your topic of interest.

GUIDE:For an author search, do "lastname firstinitials" without commas or periods. Separate multiple authors with commas.

For example, Thornhill and Ussery wrote an article outlining the various ways that "irreducibly complexity" can evolve. Search PubMed on "ussery d, thornhill" and you get:

That biological features may change their function during evolution has long been recognized. Particularly, the acquisition of new functions by molecules involved in developmental pathways is suspected to cause important morphologic novelties. However, the current terminology describing functional changes during evolution (co-option or recruitment) fails to recognize important biologic distinctions between diverse evolutionary routes involving functional shifts. The main goal of our work is to stress the importance of an apparently trivial distinction: Whether or not the element that adopts a new function (anything from a morphologic structure to a protein domain) is a single or a duplicated element. We propose that natural selection must act in a radically different way, depending on the historic succession of co-option and duplication events; that is, co-option may provide the selective pressure for a subsequent gene duplication or could be a stabilizing factor that helps maintain redundancy after gene duplication. We review the evidence available on functional changes, focusing whenever possible on developmental molecules, and we propose a conceptual framework for the study of functional shifts during evolution with a level of resolution appropriate to the power of our current methodologies.

But what else exists out there on this topic? Trying different keywords is a possibility, e.g. "cooption", "co-option", "co-optation", "change in function", "functional shift", etc., but this is tedious. Instead, once you've found one good article, click on "Related articles":

...is a central archive of scientific literature that is freely available to the public without subscription. Sometimes the whole journal is free, sometimes the material is made freely available after 6 months.

Oftentimes you will have to complete a free registration to access free content.

A list of journals with full-text access for subscribers (the subscribers are usually university libraries, generally they are available to anyone within the University's edu domain) tied into the PubMed search engine is here: